1. Field of the Invention
The present invention relates to that step in the kraft wood pulping process chemical recovery flow stream where sodium sulfide present in the expended cooking chemical (black liquor) is oxidized in a gas-liquid contact reaction environment to one of the more stable compounds of sodium thiosulfate and/or sodium sulfate. The subject process step is generically characterized in the industry as black liquor oxidation and has the purpose of preventing the release of toxic, gaseous hydrogen sulfide which, otherwise, would reactively evolve within and uncontrollably emanate from the direct contact evaporation stage of the recovery stream.
2. The Prior Art
As a technological specialty, black liquor oxidation has approximately 25 years of development. Although the basic functional concept is quite simple, e.g., intimately contacting the black liquor stream with a reactively free source of oxygen, at least six distinct approaches to the objective were identified by E. T. Guest in his "Developments In Black Liquor Oxidation" published in the December, 1965 issue of Pulp and Paper Magazine of Canada. The present invention may be classified in the E. T. Guest category of "air pressure systems."
In terms of equipment design and complexity, the air pressure system may be characterized as the most simple of the several alternatives and basically comprises an unpressurized liquor retention vessel wherein liquor is introduced at the top and withdrawn from the bottom. Simultaneously, air or oxygen is injected into the reservoir near the bottom thereof from a plurality of nozzles distributed over the vessel section. Since the bubbles of gaseous oxygen buoyantly migrate toward the vessel top, the relative gas-liquid flow is countercurrent.
J. E. Landry in his December, 1963 TAPPI Vol. 46, No. 12 presentation of "Black Liquor Oxidation Practice And Development -- A Critical Review" describes, on page 769 thereof, the operation of a classical "air pressure system" in greater detail.
U.S. Pat. Nos. 3,549,314 and 3,567,400 issued to I. S. Shah disclose hybrid modifications of the classical "air pressure system."
Operationally, the technical objectives of black liquor oxidation systems include the oxidative stabilization of sodium sulfide present in kraft black liquor by chemically transforming it to sodium thiosulfate and/or sodium sulfate. By virtue of this expedient, loss of economically valuable sulfur to the atmosphere is prevented. Without the black liquor oxidation expedient, such losses occur in the form of gaseous hydrogen sulfide and methyl mercaptans which hydrolytically evolve from the final liquor concentration step performed by the direct contact evaporator. Moreover, the compounds, hydrogen sulfide and methyl mercaptan, are highly toxic and malodorous and therefore highly objectionable to the surrounding community.
At the present state of the black liquor oxidation art development, virtually 100% of the sodium sulfide present in a black liquor flow stream may be reactively converted by an air pressure system operating at design conditions.
However, much is left to be desired in the form of efficient, normalized retention times and the operational continuance of design conditions.
The normalized retention time of a particular black liquor oxidation process and apparatus is the product of the calculated time interval required for flowing one unit of vessel volume through the system, in minutes, multiplied by the proportionality of oxygen actually supplied to the oxygen quantity stoichiometrically required to react all of the sodium sulfide present in a given unit of black liquor. Mathematically, normalized retention time may be expressed as: ##EQU1##
Prior art oxidation systems achieve 95 to 99% sodium sulfide removal with 200 to 3000% excess oxygen in 15 to 200 minutes. Depending on the performance efficiency of such prior systems, they may be judged from good to poor.
The design of a practical, efficient, black liquor oxidation system comprises numerous compromises. To flood the reservoir with large excesses of oxygen is a heavy tax against the system economics. If the oxygen is purchased in pure form, most is wasted to buy retention time. If the oxygen is derived from compressed air, the time is purchased with huge power consumptions. If large, 200 - 300 minutes, actual reaction times are used, such time is purchased with large capital expenditures on tank volume and real estate, whether in a single tank or a plurality of smaller tanks operated in series or parallel flow.
In addition to the foregoing, most prior art systems suffer nozzle plugging within a relatively short order of time after start-up. Although the total oxygen flow rate may remain relatively unchanged after plugging of 10 to 15% of the originally available nozzles, system performance falls off considerably due to the consequent, maldistribution of oxygen within the reservoir. If allowed, obstruction of up to 80% of the original nozzle area may occur within 60 days of continuous operation. Accordingly, most prior art systems are shut down each 1 to 3 days for a few hours to soak the accumulated nozzle deposits and subsequent back flushing.
While the mechanics of such nozzle plugging are not completely known, one contributing factor is the lack of uniformly adequate pressure distribution throughout the oxygen supply system. Analysis of plugging patterns in prior art distribution systems reveals a propensity of such systems to suffer initial plugging at those nozzles most proximate of the air supply line juncture with the main manifold. One explanation of this phenomenon relates to the Bernoulli function of EQU P.sub.t = P.sub.s + P.sub.d
where:
P.sub.t is total pressure PA1 P.sub.s is static pressure PA1 P.sub.d is dynamic or inertial pressure
According to the explanative theory, the air supply flow experiences a velocity increase in the juncture region of the supply line with the main manifold where, in prior art systems, the total mass flow is allowed to divide into two flow streams from the juncture point. Flow area for the two exit streams is substantially more than flow for the entrance stream. Since the total pressure P.sub.t must remain constant, an increase in the dynamic pressure P.sub.d is gained at a sacrifice in static pressure P.sub.s. However, since flow from the main header into the subheaders is transverse of the dynamic pressure force vector, the only available driving force to induce mass flow into the subheaders is the static pressure P.sub.s. Accordingly, if a moving air mass is accelerating along a subheader axis into and from a lateral subheader junction point, such acceleration increase in velocity supplements the P.sub.d parameter of the Bernoulli function at the sacrifice of P.sub.s. Consequently, relatively less mass enters the subheader thereby resulting in a lower pressure differential across the throat of those injection nozzles nearest the supply line junction.
In consideration of the aforedescribed prior art, an objective of the present invention is to improve the operational efficiency of pressurized air, black liquor oxidation systems.
Another objective of the present invention is to operationally sustain such improved performance of a pressurized air, black liquor oxidation system by eliminating nozzle plugging due to poor air distribution.
A further objective of the present invention is to teach a self-draining air distribution system having no liquid stagnation point whereby liquor which is permitted to back flow into the air system during periods of operational interruption may be purged from the air system by pressure differential and gravity upon resumption of operation.